Scanning Probe Techniques forEngineering Nanoelectronic Devices

AFM

Asylum ResearchLandon Prisbrey,1 Ji-Yong Park,2 Kerstin Blank,3 Amir Moshar,4 and Ethan D. Minot1*
1

Department of Physics, Oregon State University, Corvallis, Oregon 97331-6507. 2Deparment of Physics and Division of Energy

Systems Research, Ajou University, Suwon, Korea. 3Institute for Molecules and Materials, Radboud University, Nijmegen,
The Netherlands. 4Asylum Research, Santa Barbara, CA 93117, USA*minote@science.oregonstate.edu

Introduction
A growing toolkit of scanning probe microscopy-based techniques is enabling new ways to build and investigate nanoscale
electronic devices. Here we review several advanced techniques to characterize and manipulate nanoelectronic devices using
an atomic force microscope (AFM). Starting from a carbon nanotube (CNT) network device that is fabricated by conventional
photolithography (micron-scale resolution) individual carbon nanotubes can be characterized, unwanted carbon nanotubes
can be cut, and an atomic-sized transistor with single molecule detection capabilities can be created. These measurement andmanipulation processes were performed with the MFP-3D™ AFM with the Probe Station Option and illustrate the recent
progress in AFM-based techniques for nanoelectronics research.

Research in nanoscale systems relies heavily on scanning probe
microscopy. Many of these nanoscale systems have uniqueproperties that are amenable to scanning probe techniques
and, therefore, a wide variety of specialized scanning probe
techniques have been developed. For example, in the field ofmagnetic memory research, magnetized probes are used tomap magnetization patterns,1 while ferroelectric materials arestudied by locating the electric fields from polarized domains.2
In many cases the scanning probe microscope is used both asa manipulation tool as well as an imaging tool. It is possible,
for example, to flip discrete magnetic or ferroelectric domains
and then image the resulting changes.2,3
In the field of nanoelectronic devices, researchers havedeveloped their own unique set of scanning probe imaging
and manipulation techniques. In this article we review
key techniques that allow for mapping the distribution ofelectrical resistances, measuring local field-effect sensitivity,and engineering new electrical characteristics into nanoscale
devices.
We use CNT devices as our working example, showingthat parallely-connected CNTs can be individually analyzed,
unwanted CNTs can be cut, and finally a point defect canbe created which has single-molecule detection capabilities.
While we focus on CNT devices, many of the same techniques
are being used for nanowire4 and graphene5 devices.
We first discuss how electric force microscopy (EFM) andscanning gate microscopy (SGM) techniques are used tocharacterize the electrical response of nanoelectronic devices.
The information contained in EFM and SGM images is
extremely valuable for relating the local electrical behaviorof a device to the global device properties. In the specificexample of our CNT network device, we use the knowledge

1

Figure 1: (a) Schematic illustrating a carbon nanotube field-effecttransistor and ‘hover pass’ (see text) microscopy geometries (not toscale). (b) AFM topography colored with electric force microscopy
phase (Vtip = 8 V, height = 20 nm).

of local resistance and local semiconducting behavior tochoose the “best” CNT out of a mixture of parallel CNTs.
To acquire EFM and SGM signals, a conducting AFM probe
hovers above an electrically contacted device (Figure 1a).
During a “hover pass”, the microscope maintains a constantseparation between the conductive AFM tip and the surface

Scanning Probe Techniques forEngineering Nanoelectronic Devices
in Figure 1b and 2a are a combination of a dc-EFM data (color
scale) and AFM topography. Many individual CNTs are seen
bridging the gap between the source and drain electrodes.
We are interested in learning which CNTs are metallic, which
are semiconducting, which are well connected to the metalelectrodes and which contain natural defects.

Table 1: Key differences between modes used for imaging
nanoelectronic devices.

(typical separations are 20 - 200 nm). To achieve this constantseparation distance, the microscope alternates between standard
topographical line scans and hover pass line scans. A typicalelectrical contact configuration involves a current amplifier andthree voltage sources (Vsd, Vtip, and Vbg). Depending on theparticular imaging technique, different voltage signals will beapplied and the AFM tip may or may not be piezoelectricallydriven (see Table 1).
To demonstrate the utility of EFM and SGM, we used dc-EFM,6
ac-EFM,7 SGM,8 and tip modulated SGM (tm-SGM)9 to image acarbon nanotube (CNT) network device. The device is fabricatedby growing CNTs on a SiO2 / Si substrate (300 nm oxide),10 andthen patterning electrodes with photolithography. The images

The electrostatic forces which give imaging contrast in dc-EFM6(Figure 1b and 2a) are due to a voltage difference between the
tip and the grounded electrodes. During each hover pass, theAFM cantilever is piezoelectrically driven at a fixed frequency(the fundamental resonance of the cantilever). If the AFM tip isabove a grounded conductor, a vertical gradient in electrostaticforce causes a shift in the resonance frequency of the cantilever.
This resonant frequency shift modulates both the phase andamplitude of the cantilever motion. The dc-EFM image shown in
Figure 2a allows us to quickly locate all conducting objects in thelarge (60 μm) imaging area and requires minimal setup.
The vertical gradient in electrostatic force, which gives rise to
the dc-EFM signal, depends on the tip-sample capacitanceand the tip-sample voltage difference. When the tip hovers
above a large-area electrode, the vertical gradient in tip-sample
capacitance is large. When the tip hovers over a small CNT, thevertical gradient in tip-sample capacitance is small. Thus, theFigure 2: ‘Hover pass’ microscopy data of a typical CNT FET. (a) EFM (Vsd = 20 mV, Vbg = 0 V, Vtip = 5 V, height = 20 nm). Color scale shows phaseof cantilever response. (b) Alternating current EFM (Vsd = 100 mV @ 63 kHz, Vbg = 0 V, Vtip = 2 V, height = 20 nm). Color scale shows amplitude of
cantilever response. (c) SGM (Vsd = 20 mV, Vbg = 0 V, Vtip = 5 V, height = 20 nm). Color scale shows the change in current, ΔIsd , through the device.
(d) Tip-modulated SGM (Vsd = 20 mV, Vbg = 0 V, Vtip = 5 V, height = 20 nm). Color scale shows the change in current at the cantilever fundamental
frequency, ΔIsd,ac .

2

Scanning Probe Techniques forEngineering Nanoelectronic Devices
image. Cantilever oscillations are strongest when the probe isnear the biased electrode. The ac-EFM signal is “cleaner” thandc-EFM signals because stray DC signals (such as charge on theinsulating substrate) are invisible in ac-EFM.
We now turn our attention to scanning gate techniques,
which allow researchers to further characterize the electronic
properties of individual CNTs. Scanning gate techniques identify
the local semiconducting response of a nanoscale conductor,
revealing the presence of localized semiconducting ‘hotspots’and bottlenecks for electron transport.8 Figure 2c shows an SGM
image, where the AFM probe acts as a roaming gate electrodeto modulate the doping in the underlying electronics. The SGM
image (Figure 2c) shows a single feature, indicating that the
global semiconductor response of the device is dominated bya single CNT (labeled 10 in Figure 2b). Other CNTs are present
in the imaging area, but the SGM signals from these are
significantly smaller. Methods to improve sensitivity are discussed
below.
Figure 3: Electrical cutting of unwanted CNTs. (a) EFM image of a device.
Cutting path indicated by dotted arrow (Vsd = 25 mV, Vbg = 0 V,
Vtip = 8 V, height = 20 nm). (b) Time trace of cutting event (Vsd = 25 mV,
Vbg = 0 V, Vtip = -8 V, contact force = 6-9 nN).

EFM signal from CNTs is weaker than from the electrodes. There
has been steady progress in mapping out the vertical gradientin tip-sample capacitance to allow more quantitative imaging of
the potential difference between the tip and the sample.11,12

The imaging contrast in Figure 2c corresponds to changes in
global device current when the biased probe locally changes thedoping level in the underlying electronics. A small source-drain
voltage (Vsd ) is applied across the device to drive global current.
By monitoring changes in current as a function of tip position,
the local field-effect sensitivity can be mapped.

Figure 2b shows an ac-EFM7 image in which different voltage
signals are sent to the source and drain electrodes, respectively.
This type of ac-EFM scan is used to visualize the voltage
profile along the length of electrically-biased CNTs. A similar
measurement can be done with dc-EFM, however, the ac-EFM
signal is cleaner (more discussion below). CNTs labeled 1-4 inFigure 2b exhibit a sharp voltage drop where they meet the
inner electrode (the length of the CNT appears uniform black).
This sudden voltage drop indicates a high contact resistance.
The CNT labeled 5 exhibits a sharp voltage drop midway along
its length. The location of the voltage drop corresponds to a
sharp bend in the CNT, showing that the bend is a point of highresistance. CNTs labeled 6-10 show a gradual voltage gradientalong their length, indicating that these CNTs are well connected
to both electrodes, and are free from major electrical defects.
These CNTs with small contact resistance will be subsequently
singled out for additional engineering and experiments.
To obtain the ac-EFM signal in Figure 2b, an AC bias was applied
to the outer electrode (frequency matching the cantilever’s
mechanical resonance) while the inner electrode and the AFM
tip were grounded. During the hover pass the cantilever is
not piezoelectrically driven, instead cantilever oscillations are
driven by capacitive coupling to the AC biased electrode. Theseoscillations are detected by monitoring the cantilever deflection
signal using a lock-in amplifier. The oscillation amplitude isplotted as a function of tip position to create the ac-EFM

Figure 4: Defect engineering in a CNT FET. (a) Schematic illustrating
defect engineering geometry. (b) Transistor characteristics before(dashed black) and after (solid red) defect creation. The red curve has
been multiplied by 10 (Vsd = 25 mV). (c) AFM topography of a CNT
device. A defect was engineered at the location indicated by an arrow.
(d) SGM data of the device shown in (c) taken before and (e) after
defect engineering (Vsd = 25 mV, Vbg = 0 V, Vtip = 8 V, height = 20 nm).
The color scale (ΔIsd / I0 ) indicates changes in device current relative
to the baseline value, I0 . In (d) and (e) the values of I0 are 280 nA and
7 nA, respectively.

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Scanning Probe Techniques forEngineering Nanoelectronic Devices
A variation of SGM which offers greater sensitivity is tipmodulated SGM (tm-SGM).9 Compared to conventional SGM,
tm-SGM offers higher spatial resolution and is capable ofresolving weaker signals. Figure 2d shows a tm-SGM image. This
scan reveals five CNTs with a semiconducting response (labeled
6-10 in Figure 2b). One CNT contains a region of highly localized
semiconducting response, indicating the presence of a naturaldefect (labeled 8 in Figure 2b).
In tm-SGM the AFM cantilever is biased and piezoelectricallydriven at its resonant frequency, resulting in an AC electricfield which modulates the local doping level in the underlying
electronics. The local field-effect response of the sample ismapped as a function of tip position by isolating the componentof the conductance signal at the cantilever resonant frequency
using a lock-in amplifier.
The detailed characterization outlined in Figure 2 allows
us to pick the best CNT out of the ten CNTs for building ananotransistor with chemical functionality. Using dc-EFM (Figure2a), we identified the locations of all CNTs in a large area scan,
providing a road map for engineering. With ac-EFM (Figure 2b)
we identified desirable CNTs with low contact resistance (nosudden drop in voltage at metal-CNT contact) and an absence of
natural defects (no sudden drop in voltage along length of CNT).
Finally, using SGM methods (Figures 2c and 2d), we identified
CNTs with a semiconducting response. The most desirable CNT
for building a nanotransistor is one with low contact resistance,
no natural defects, and a uniform semiconducting response
along its length. The CNT labeled 10 in Figure 2b satisfies thesecriteria, and is singled out for further experimentation.
With initial characterization complete, we move to AFM
manipulation techniques. Once a desired CNT is identified, we
use AFM engineering to prune unwanted CNTs,13 then engineer
an atomic-sized transistor element into the one remaining
CNT.14 These techniques combine AFM lithography modes with
control of the tip-sample voltage. Mechanical manipulation
by AFM has also been used by many authors,15,16 but is notdiscussed here.
To cut unwanted CNTs, a Pt-coated AFM probe is biased toVtip = -10 V, engaged with the surface, and then dragged
through the CNTs in contact mode (contact force ~ 6-9 nN).
Previously obtained dc-EFM and ac-EFM scans, such as in Figures
2a and 2b, provide a convenient road map of the locations of
the unwanted CNTs. Figure 3 shows a time trace of global device
conductance during electrical cutting. Sharp decreases in device
conductance are observed as the CNTs are severed. Afterward,
ac-EFM and SGM scans may be used to verify that a single CNT
forms the only electrical contact between the electrodes.
A single CNT can be converted into a transistor by further AFMbased engineering. Such nanotransistors approach the ultimate
limit of miniaturization. To engineer an atomic-sized transistor,
a conducting AFM probe is brought into gentle contact with the
CNT and a -3 V, 15 ms square wave pulse is applied to the tip

4

Figure 5: Single-molecule detection of EDC. (a) Schematic illustrating
detection geometry. The liquid potential is controlled with a liquid-gate
electrode biased to Vlg. (b) Isd trace of a nanotransistor in MES buffer(pH 4.5, Vsd = 25 mV, Vlg = 250 mV). (c) Isd trace of the same device
following the addition of 20 μM EDC (pH 4.5, Vsd = 25 mV, Vlg = 100 mV).

(Figure 4a). During the pulse, a chemical defect is incorporated
into the sidewall of the CNT, and the device’s electrical properties
change significantly as a result (Figure 4b).14,17 Typical defects
add between 10 kΩ to 1 MΩ to the overall resistance of thecircuit, and change the field-effect sensitivity of the device.
Figure 4c shows a single-CNT device produced by the electrical‘nicking’ method described above. SGM measurements showthat the pristine CNT is uniformly gate sensitive (Figure 4d).
Figure 4e shows an identical measurement following defectengineering where gate sensitivity is localized to the region
around the defect. The SGM scan reveals that the defect acts
as a gate sensitive bottleneck for transport, and the remainderof the CNT serves only as contact electrodes to the miniaturetransistor.
A unique attribute of point-defect nanotransistors is their
sensitivity to electrostatic potential within a very small detection
volume. As such, they are ideally suited for single-moleculesensing applications. Recently, point-defect nanotransistor
sensors have been used to study biochemical reactions at thesingle molecule level.18,19
Figure 5 demonstrates the use of an AFM-engineered pointdefect nanotransistor as a single-molecule sensor. The reaction
between N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide(EDC) and a carboxyl group is used as a model reaction. EDC isfrequently used to activate carboxyl groups in bioconjugation
chemistry, and reacts reversibly with a carboxyl point defecton the CNT sidewall. The point-defect nanotransistor was
immersed in an electrolyte solution containing EDC (Figure 5a).
Figures 5b and 5c show the conductance data for a point-defect
CNT before and after the addition of 20 μM EDC, respectively.
Discrete switching events are observed in the presence of EDC.

Scanning Probe Techniques forEngineering Nanoelectronic Devices
This two-state telegraph noise is believed to reflect changes inthe electrostatic environment resulting from the reaction withsingle EDC molecules.18 When EDC is bound to the defect,
current through the CNT is low. When EDC releases from thedefect, current through the CNT is high.

Summary
Using CNT network sensors as our working example, we havereviewed AFM-based techniques which are used to study andengineer nanoelectronic devices. We have used dc-EFM andac-EFM to identify the locations and resistances of individual
CNTs that are electrically connected in parallel. Next, SGM andtm-SGM were used to reveal the semiconducting response ofeach CNT.

Figure 6: The Probe Station attaches to the MFP-3D scanner and allows
easy electrical probing of sample properties, electrical biasing, and othermeasurements while the sample is being scanned with the AFM. A
variety of electrical connections can be made and combined with various
imaging modes.

With the information available in these scans a single CNT withdesirable properties was singled out for further experimentation.
The unwanted CNTs were electrically cut with a biased AFM
probe to leave a device containing a single CNT. An atomic-sizedtransistor with chemical functionality was engineered in theremaining CNT using a voltage pulse from the AFM probe. Thisnanotransistor was then demonstrated to be a single molecule
sensor sensitive to EDC in an aqueous environment. This multistep measurement and manipulation process illustrates thepower of AFM-based techniques to map out and control theproperties of nanoelectronic devices.

Acknowledgements
We thank Kristina Prisbrey for assistance with Figures 1, 4,
and 5, and Bo Criss for assistance with Figure 2. We thankSophie Ripp for initial experiments with EDC using suspended
CNTs. We thank Paul Schuele and Sharp Labs of America forassistance with device fabrication. This work is funded by the
Human Frontier Science Program. All AFM measurements andmanipulations described here were performed with the
MFP-3D AFM (Asylum Research, Santa Barbara, CA,
www.asylumresearch.com) with the Probe Station Option(Figure 6).

5

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